Safety Light Curtain Wiring Diagrams — The Complete Engineering Guide (DQC & All Series)
In industrial automation production lines, the correct wiring of safety light curtains not only ensures normal equipment operation but also serves as the first line of defense for operator safety. This guide collects 32 labeled wiring diagrams in one place: 5-core and 7-core color codes, NPN and PNP hookups, dual-channel OSSD with EDM feedback, safety relay and PLC connections, brand-replacement mapping, the seven wiring mistakes we see most in the field, and the commissioning trip test. As a leading domestic sensor manufacturer, DAIDISIKE (戴迪斯科) DQC series universal safety light curtains have gained market recognition for their high reliability and simplified installation process — the DQC 5-core method opens the guide, followed by every other series.
All 32 diagrams from this guide in one printable file: color codes, NPN/PNP hookups, OSSD + EDM loops, relay and PLC wiring, the seven costly mistakes, and the commissioning test sheet.
Which wiring diagram do you need? Four questions that decide the circuit
Every diagram further down this page is one combination of four answers. Settle these first and you will know which figure to scroll to instead of reading all thirty-two.
- NPN or PNP? This is fixed by the model suffix at the point of order, not by wiring. On DQC and DQA the fourth field of the part number carries it: A = NPN normally-closed, B = PNP normally-closed, C = NPN normally-open, D = PNP normally-open, and on DQA J = volt-free relay output instead of a transistor. Read the nameplate before you plan the panel — a DQC08/40-2801AA is an NPN NC unit and will not drive a sinking input correctly.
- Safety relay, safety PLC, or a curtain with a built-in controller? A transistor-output curtain always needs something to evaluate its two channels. A DQA ordered with the J suffix, or a DQC paired with the DQCA / DQCA2 controller, already contains the relay stage.
- Is EDM used or bridged? External Device Monitoring watches the contactors for welded contacts. If your safety relay supports it and you are switching contactors, wire it. If you are not using it, bridge the terminal per the relay manual — never leave it floating.
- Automatic, manual or monitored reset? Automatic restart is only permissible where nobody can stand inside the guarded zone undetected. Anything else needs a reset button mounted outside the guarded area with a clear view of it.
Transmitter or receiver? Identifying an unlabelled unit with a meter
The single most common real-world starting point is a curtain already bolted to a machine with the label worn off and no manual in the cabinet. None of the wiring guides currently ranking for this query address it. Here is the procedure, in the order that keeps you safe.
Telling the transmitter from the receiver
On DQC and DQA the two sticks are not interchangeable and differ in three observable ways. The transmitter carries fewer conductors — the DQC datasheet specifies four cores at the projector and five at the receiver, because only the receiver has an output to send anywhere. The indicator behaviour differs: the transmitter has a single red power lamp that lights as soon as it is energised and never changes, while the receiver's indicator is the diagnostic one — green when every beam is clear, red when the field is broken. Finally the receiver is the one whose state changes when you pass your hand through the field. If nothing on a stick reacts to an interruption, it is the transmitter.
Finding 0 V and +24 V safely
Work de-energised first. With the supply isolated and locked off, use the continuity range to find which conductor is bonded to the panel earth stud — on DAIDISIKE cable that is the yellow-green shield, and it should read near zero ohms to PE. Then restore power and, with the meter on DC volts and the black probe on the panel 0 V rail, measure each remaining conductor in turn. The supply pair is unmistakable: one conductor sits at roughly +24 V and does not move regardless of whether the field is clear or blocked, and one sits at 0 V. On DAIDISIKE cable these are brown and blue respectively.
Distinguishing an output from the sync conductor
Of the conductors that remain, the output changes state when you break the beam and the sync line does not. On the DQC the synchronisation conductor is black, labelled CP, and it is butt-jointed directly between transmitter and receiver — it goes to the other stick, not to your panel, so if you trace a conductor and it terminates at the opposite unit you have found CP. The white conductor is the one that switches. On a six-core DQC dual-output unit there are two switching conductors: white is OUT1 and grey is OUT2.
I. DQC Series 5-Core Cable Color and Function Definitions
DQC series safety sensors connect via aviation connectors to M12 five-core cables. Before wiring, strictly verify cable colors and their corresponding electrical functions to prevent circuit burnout from reverse polarity.
| Cable Color | Signal Definition | Function Description |
|---|---|---|
| Brown | +12~24VDC | Power Positive |
| Blue | 0V / GND | Power Negative |
| Black | CP (Control Signal) | Synchronization signal line between transmitter and receiver |
| White | OSSD Output | Safety switch signal output (connects to relay or PLC) |
| Yellow-Green | Shielding Wire | Suppresses electromagnetic interference; connects to ground |
II. DQC 5-Core Wiring Diagrams — NPN and PNP
The DQC series typically supports both NPN and PNP logic output modes (output logic follows the A/B/C/D model suffix). The NPN single-output connection is the most common, so it comes first; the PNP variant differs only in where the load sits.
1. Transmitter Wiring
The transmitter primarily emits the infrared beam. The brown wire connects to the 24VDC positive terminal, and the blue wire connects to 0V. The black wire (CP) must be connected to the receiver's black wire via the interface to achieve optical axis synchronization.
2. Receiver Wiring
The receiver detects signals and outputs logic based on light flux status. The brown wire connects to 24VDC, and the blue wire to 0V. The white wire (OSSD) is the core output line, typically connected to a safety relay (e.g., K1 load) to control motor disconnection.
Same drawings, three search names
Engineers arrive at this library searching for a safety light curtain wiring diagram, a light curtain sensor wiring diagram, or simply a light curtain wiring diagram — they are the same drawings. Whether your paperwork calls the device a safety light curtain, a light curtain sensor or a light barrier, the cable colors, OSSD outputs and relay hookups on this page apply unchanged; only the model table you order from differs.
III. Wiring by Series — Cores, Colors and Synchronization
Different DAIDISIKE series use different cable architectures: the economical JER transmitter needs only 3 cores because synchronization is optical, while the Type 4-architecture DQT4 runs a 9-core cable for its redundant PNP OSSD pair with EDM. The chart below is the fastest way to know what you will find in the junction box before the panels are even open.
IV. OSSD Dual Channels and the EDM Feedback Loop
Everything downstream of the receiver depends on two rules: the two OSSD channels stay fully independent (own fuse, own routing), and the contactors report back through EDM so a welded contact blocks the restart. The two diagrams below are the heart of this entire guide — if you internalize them, every relay and PLC hookup that follows is just detail.
Self-test pulses — and why a standard PLC input misreads them
An OSSD does not simply sit high while the field is clear. It briefly pulls itself low, over and over, and watches whether the line actually follows. That is how the curtain detects a short to +24 V, a short to 0 V or a short between the two channels — faults that a simple on/off output could never reveal. A safety input expects those pulses and ignores them. A standard PLC input often does not: if its filter is faster than the pulse, it reports every self-test as a real interruption, and you get phantom transitions and nuisance trips on a curtain that is behaving perfectly.
V. Connecting to a Safety Relay, a PLC, and the Reset Circuit
The canonical downstream hookup is a dual-OSSD curtain into a safety relay such as the DAIDISIKE DA31 (3 force-guided NO contacts + 1 NC auxiliary, release < 30 ms). For deeper treatment of each leg, this hub links out to the dedicated guides: safety relay wiring in detail, NPN/PNP output logic and PLC integration, and the installation & wiring overview.
When you don't need a separate safety relay — relay-output and built-in-controller curtains
Not every installation needs a DIN-rail safety relay. Two DAIDISIKE options put the relay stage inside the product, which removes a component from the panel and a pair of terminations from the job.
DQA with the J output suffix — volt-free relay contacts
Ordering a DQA with J in the signal-output field replaces the transistor output with an internal relay giving volt-free contacts rated AC 250 V 10 A and DC 30 V 16 A. The cable gains three function conductors in place of the single transistor output:
- Red — common (COM)
- Green — normally-closed contact
- White — normally-open contact
The contact behaviour is the part people get wrong. With the field clear, red-green is closed and red-white is open. With the field blocked, red-green opens and red-white closes. The stop function therefore rides on the red-green pair, because that is the pair that opens on detection and on loss of power — the fail-safe direction. Brown, blue and the black CP sync line are unchanged from the transistor version.
DQC with the DQCA or DQCA2 controller
The DQC series can be supplied with an external controller — DQCA for single-sided systems, DQCA2 for double-sided — which accepts the curtain and provides mains-voltage input (AC 110 V / 220 V) plus relay outputs, so the curtain can drop a machine circuit without a separate 24 V safety relay in the panel.
VI. Replacing Omron, Keyence, SICK or Pilz Curtains — Wiring Maps
Most rewiring jobs we support are brand swaps on running machines. The safe method is always the same: match functions, never colors — similar M12 connectors do not guarantee identical pinouts. Full comparisons live on the dedicated pages for Omron F3SG-SR, Keyence SL-V / GL-R and SICK deTec / Pilz PSENopt.
VII. The Seven Wiring Mistakes That Cost the Most
Each of these has reached our support inbox more than once. They all pass a quick bench test — and fail on the machine, sometimes silently. The full write-up lives in the OSSD & EDM common-mistakes guide.
VIII. Mounting Distance and the Commissioning Trip Test
Wiring is only finished when the curtain is mounted at the ISO 13855 distance and the trip test is on record. A perfectly wired curtain mounted too close to the hazard still fails the risk assessment.
IX. Importance of the Shielded Wire (Yellow-Green)
The DAIDISIKE DQC series is specially equipped with a yellow-green shielded cable to handle complex electromagnetic environments.
- Interference Resistance: Suppresses electromagnetic interference when reliably grounded.
- Preventing False Alarms: Failure to connect the shielded wire may cause momentary false alarms from surges.
- Connection: Bundle the yellow-green wires and connect to the factory's common ground (PE) or 0V terminal.
X. Cable Selection, Run Length and Voltage Drop
The most common cause of a curtain that passes a bench test and then faults on the machine is not the curtain — it is the cable. A light curtain pair draws roughly 300 mA, and every metre of conductor between the cabinet and the receiver drops a little of the 24 V it needs. IEC 61131-2 sets the floor at 24 V −15%, or 20.4 V; below that the receiver's internal supervision starts declaring faults that look exactly like optical problems.
The drop is a round trip — current goes out on brown and back on blue — so a 40 m installation means 80 m of copper. With annealed copper at 0.0175 Ω·mm²/m, the usable run length falls out directly from conductor cross-section:
| Conductor | Resistance | Max run @ 300 mA | Typical use |
|---|---|---|---|
| 0.25 mm² (AWG 24) | 0.070 Ω/m | 25 m | Bench work and in-panel jumpers only |
| 0.34 mm² (AWG 22) | 0.052 Ω/m | 35 m | Standard factory cordsets — most installs |
| 0.50 mm² (AWG 20) | 0.035 Ω/m | 50 m | Long guard perimeters, palletizer cells |
| 0.75 mm² (AWG 18) | 0.023 Ω/m | 80 m | Cross-plant runs, outdoor perimeter |
| 1.00 mm² (AWG 17) | 0.017 Ω/m | 105 m | Long-range through-beam, remote cabinets |
Three rules that are not optional. Never run OSSD conductors in the same duct as motor or VFD cabling — the switching transients couple straight into the safety channel and produce trips no one can reproduce. Never extend a cable with a twisted joint in a junction box; use a moulded cordset or a proper terminal. And where a run exceeds the figures above, move the 24 V supply closer rather than accepting the drop — a local DIN-rail supply is cheaper than a week of intermittent-fault hunting.
XI. M12 Connector Pinouts and Cordset Selection
Flying-lead curtains are wired by colour; connectorised curtains are wired by pin number, and the two do not always agree. The pin assignment below is fixed by IEC 61076-2-101 for A-coded M12 and is the same on every compliant device. The colours are the conventional cordset colours — usually right, never guaranteed.
| Pin | Cordset colour | 4-pin | 5-pin | Notes |
|---|---|---|---|---|
| 1 | Brown | +24 VDC | +24 VDC | Supply — same on both variants |
| 2 | White | OSSD 2 / input | OSSD 2 | Second safety channel |
| 3 | Blue | 0 V | 0 V | Supply return — same on both |
| 4 | Black | OSSD 1 | OSSD 1 | First safety channel |
| 5 | Grey | — | EDM / test / select | Only on 5-pin; function is model-specific |
Two practical points. First, A-coded is for signals, not power — if you find a B, D or L-coded connector on a guard, it is a fieldbus or a motor feed and it does not belong on the safety circuit. Second, the pin-5 function is the one that varies between manufacturers: on some models it is EDM feedback, on others a test input or a range-select line. On a brand swap this is the single most likely pin to be wrong, which is why section VI above insists on matching functions rather than pin numbers.
XII. Muting and Blanking — Wiring the Exceptions
Muting and blanking both let material through a guard that would otherwise stop the machine, and they are wired very differently. Getting the distinction wrong is one of the few wiring errors that can silently remove protection while every LED stays green. The full decision treatment lives in muting vs blanking; what follows is the wiring.
Muting suspends the whole protective field for a defined window while a pallet passes. It requires at least two independent muting sensors arranged so that no single object — least of all a person — can trigger them in a valid sequence. The standard arrangements are crossed-beam (two photoelectric sensors whose beams intersect at the guard line) or the four-sensor T/L pattern for bidirectional conveyors. The muting inputs go to the safety relay or muting module, never to the curtain itself, and the sequence logic enforces both an order and a maximum time. A muting window that never times out is not muting — it is a bypass.
A muting indicator lamp is mandatory, not a convenience: it tells anyone approaching that the guard is currently suspended. Most safety relays provide a dedicated lamp output with filament monitoring, and where they do not, the lamp must be separately supervised.
Blanking is different — it disables specific beams permanently (fixed blanking, for a fixture that always occupies the field) or lets a defined object move through a window of beams (floating blanking). Blanking is configured on the curtain, not wired externally, and it carries a hard consequence: blanking beams degrades the effective resolution. A 14 mm finger-protection curtain with two beams blanked no longer detects a finger at that height, and the ISO 13855 safety distance must be recalculated against the new resolution. Run the numbers again in the safety distance calculator before the machine goes back into production.
XIII. Cascading and Multi-Curtain Systems
Guarding a cell on three sides raises a question the datasheets rarely answer: one relay per curtain, or all curtains into one relay? Both are valid; they trade cost against diagnostics.
- Series OSSD into one relay. OSSD 1 of curtain A feeds the first input, OSSD 2 feeds the second, and the next curtain is wired in series in the same channel. Cheapest, and acceptable for a cell where any interruption should stop everything. The cost is diagnostic: the relay reports a stop, not which curtain caused it, which turns a five-second fault into a walk around the cell.
- One relay per curtain, contacts in series. Each curtain gets its own relay; the safety contacts are then series-wired into the contactor coil. More hardware, but every curtain reports its own state and a fault is located instantly. This is the right answer for anything with more than two guarded sides or where downtime is expensive.
- Safety PLC or safety fieldbus. Above roughly four devices the wiring itself becomes the reliability problem, and a safety controller pays for itself. See safety relay vs safety PLC for where that threshold sits.
Response times add up. The stopping-performance figure in an ISO 13855 calculation is the whole chain, not the curtain alone: curtain response + relay release + contactor drop-out + the machine's own stopping time. A 15 ms curtain behind a 30 ms relay and a 40 ms contactor is an 85 ms chain before the machine has begun to stop. Series-cascading several curtains adds each stage's response as well — recalculate the safety distance for the assembled system, never from the curtain datasheet.
XIV. Symptom-Based Wiring Troubleshooting
Most commissioning faults resolve to one of six patterns. Read the LED states first — the curtain is usually telling you exactly what is wrong before any meter comes out.
| What you see | Most likely cause | What to do |
|---|---|---|
| Receiver red, transmitter green, beam path clear | Optical misalignment, or transmitter and receiver swapped end-for-end | Re-aim until the alignment LED goes solid; confirm the transmitter is the unit with no OSSD wires |
| OSSD will not turn on, EDM lamp lit | EDM loop open — a contactor has welded, or the feedback contacts are wired NO instead of NC | Measure continuity across the feedback path with the contactors de-energised; it must be closed |
| Machine stops intermittently, no one in the guard | Optical interference from a second curtain, a reflective surface, or welding arc flash | Set adjacent pairs to opposing scan codes, add anti-reflection baffles, re-check the 130 mm reflective-surface clearance |
| OSSD toggles rapidly at power-up then settles | Normal OSSD test pulsing being read by a standard PLC input as a real transition | Route OSSD to a safety relay or safety input card; a standard input must be monitoring-only |
| Curtain works on the bench, faults on the machine | Voltage drop over the installed cable run, or 0 V referenced to a different supply than the relay | Measure supply at the receiver under load — below 20.4 V, increase conductor size per the table above; bond all 0 V to one point |
| Both channels stuck on even with the beam blocked | OSSD pair bridged, or a single OSSD paralleled into both relay inputs | Stop production immediately. Each OSSD must land on its own relay input; this defeats the dual-channel architecture |
The last row is the one to take seriously. Bridging the two OSSDs, or feeding one OSSD into both relay inputs, produces a system that looks completely normal — green LEDs, machine runs, guard appears to work — while the dual-channel architecture that earns the Type 4 / PL e rating no longer exists. A single fault will then go undetected. If you find this on a running machine, stop it. Deeper diagnostics for the optical failure modes are collected in failure modes and false trips and EDM lockout diagnosis.
What should the multimeter read? Expected voltages, clear and blocked
Measure with the black probe on the panel 0 V rail and the meter on DC volts, with the machine isolated so an unexpected start cannot injure anyone. "Clear" means every beam unobstructed and the receiver indicator green; "blocked" means the field interrupted and the indicator red.
| Conductor | Field clear | Field blocked | What it tells you |
|---|---|---|---|
| Brown (supply +) | +24 V DC | +24 V DC | Unchanging. If it sags when the field is broken the supply is undersized or shared with an inductive load. |
| Blue (0 V) | 0 V | 0 V | Any standing voltage here means a shared or broken 0 V return — fix before going further. |
| White — PNP output, NO type | ≈ +24 V (within ~1.5 V of supply) | ≈ 0 V | A sourcing output pulls up to supply less its saturation drop. The DQC/DQA spec allows up to 1.5 V, so ~22.5 V on a 24 V rail is healthy, not faulty. |
| White — NPN output, NO type | ≈ 0 V (within ~1.5 V of 0 V) | Floats to near supply | A sinking output only pulls down. Read it with the load connected — measured open-circuit it floats and the numbers mean nothing. |
| Black (CP sync) | Not a steady DC level — do not judge it by voltage | CP is butt-jointed between transmitter and receiver. Verify it with continuity between the two units, de-energised. | |
| Yellow-green (shield) | ≈ 0 V, and near 0 Ω to PE at its single bonding point | Continuity to PE at both ends means a ground loop — land it at one end only. | |
| Red–green pair, DQA relay (J) version | Closed (continuity) | Open | Volt-free contacts, so measure continuity, not volts. Red-green is the normally-closed pair and is the one that carries the stop function. |
| Red–white pair, DQA relay (J) version | Open | Closed (continuity) | The normally-open pair, for signalling and indication. Contact rating is AC 250 V 10 A / DC 30 V 16 A. |
Why won't my safety light curtain reset? An ordered diagnostic path
Work down this list in order and stop at the first step that fails. The sequence matters: each check assumes the ones above it passed, and jumping to the middle is how an afternoon disappears.
- Transmitter powered? Its red lamp should be lit the moment the supply is on. No lamp means brown/blue, not the safety circuit.
- Receiver powered? Same check on the receiver's own supply pair.
- Alignment and synchronisation. A receiver that never goes green with a genuinely clear field is usually misaligned or has lost CP. Confirm the black conductors of the two sticks are joined, then align — the DQC tolerates an incidence angle of ±5° at 10,000 lux, so a stick rotated in its bracket will not lock on.
- Reset mode. If the relay is strapped for automatic restart it will never respond to a button, and if it is strapped for manual it will never restart on its own. Confirm which mode the strapping actually selects before suspecting the button.
- Reset contact polarity and edge. Most monitored reset circuits act on the release of the button, not the press. A button held closed, or wired normally-closed into an input expecting a rising edge, produces exactly the "nothing happens" symptom.
- EDM loop continuity. With the machine de-energised, check continuity through the normally-closed auxiliary contacts of every contactor in the loop. One contactor left out of the chain, or an auxiliary block that was never fitted, opens the loop permanently. Full worked examples are in EDM feedback wiring examples with series NC contacts.
- Welded contactor. If the EDM loop is wired correctly and still open with everything de-energised, a main contact has almost certainly welded — which is precisely the failure EDM exists to catch. See EDM lockout that will not reset. Replace the contactor; do not bridge the loop to clear the fault.
- Channel discrepancy. Safety relays require both inputs to change within a short window. If one channel is slow, intermittent or open, the relay latches out. Measure both outputs per the table above.
- Cross-fault. An output shorted to +24 V looks permanently "safe to run" to a meter but is detected and latched by the relay. Inspect for crushed cable at cable entries and for a conductor caught under a terminal screw.
XV. Selection Parameters and Recommendations
Verify your requirements against these DQC specifications:
- Response Time: ≤15ms for rapid safety disconnection.
- Protection Rating: IP65-rated housing, dustproof and waterproof.
- Resolution: Choose Finger (10/14mm), Palm (20/25/30mm), or Full-Body (40/80/200mm) protection.
Standards this page works to — and who may sign the installation off
Wiring a light curtain correctly is necessary but not sufficient. The documents below govern whether the resulting protective device is actually fit for the risk it is guarding against:
- IEC 61496-1 / -2 — the product standard for electro-sensitive protective equipment, and the origin of the Type 2 versus Type 4 distinction. See Type 2 vs Type 4 explained.
- ISO 13849-1 — Performance Level and Category for the whole safety function, curtain plus relay plus contactors, not the curtain alone. See Performance Level (PL) vs SIL.
- IEC 62061 — the SIL route to the same goal.
- ISO 13855 — the minimum mounting distance, which depends on the response time of everything in the stop chain. Use the ISO 13855 safety distance calculator.
- EN 60204-1 — machine electrical equipment: the 24 V PELV/SELV supply, protective bonding and the single-point earth this page assumes throughout.
XVI. Summary
Correctly completing the wiring for a safety light curtain is the first step in ensuring workshop safety. Always remember: the CP synchronization wire must not be left floating, the shielded wire must be grounded at a single point, the OSSD channels stay independent, and the EDM loop is what stands between a welded contactor and an unplanned restart.
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